This invention relates generally to controllably bendable tube assemblies, and is more especially directed to a hollow steering section of a borescope or endoscope.
An endoscope is generally characterized as an elongated flexible tube with a viewing head in its distal or forward end, and a control housing at its proximal end for controlling or steering the distal end. Such an endoscope has a bendable-tube steering section at the distal end adjacent to the viewing head. One or two pairs of control cables extend through the bendable-tube steering section and through the remainder of the flexible tube, and these cables connect with a steering control in the control section. One or both pairs of these cables are displaced to bend the bendable-tube steering section to facilitate the inspection of an object.
An endoscope is typically inserted into a body cavity of a patient to investigate visually the tissues within the cavity. For example, an endoscope can be inserted into the colon or stomach, or into the lung of a patient. Because the esophagus, bronchii, and the colon are narrow, tortuous passageways, the steering section must be bent rather precisely, and as close to the head as possible, in order to obtain the necessary penetration without damaging the patient's tissues. It is most desireable that the play or slack in the cable be kept to an absolute minimum, so that steering can be controlled precisely.
A borescope is a similar device, but intended for visual inspection of a mechanical device, such as a jet engine or turbine, where it would be difficult or impossible to examine the device's internal elements. The borescope needs to go into narrow tortuous passageways, and must observe similar steering and bending considerations.
A number of types of steering mechanisms are known. For example, helically coiled strips are employed in endoscopes or borescopes as described in U.S. Pat. Nos. 3,610,231 and 3,739,770. Steering sections having thin-walled cylindrical segments or bands that are joined by means of pins or bifurcations or other similar articulations such that the segments are rockable on one another, are described in U.S. Pat. Nos. 3,583,393; 3,669,098; 3,799,151; and 4,347,837. A previously-proposed endoscope that had a provision to control the degree of bending is described in U.S. Pat. No. 3,557,780.
The steering mechanisms for these previously-proposed endoscopes are rather elaborate structures, with many parts that can fail and which are relatively expensive to produce. Further, in many cases it has been necessary to provide the cables with a significant amount of slack or play because the steering sections bend at discrete points, and not in a perfectly smooth curve.
U.S. Pat. No. 4,700,693, Oct. 23, 1987, and having a common assignee herewith, addresses the above problem. The disclosure in that patent is incorporated herein by reference.
In the steering section of the endoscope or borescope described in U.S. Pat. No. 4,700,693, the steering section has, within its flexible sheath, a plurality of axially aligned washers, each having a central passage and a number of peripheral bores. Pairs of these peripheral bores are disposed generally diametrically opposite each other. The steering cables pass through the respective axially aligned peripheral bores of the washers, and spacing structure is disposed at the location of predetermined ones of these peripheral bores to define bending locations for the steering section, such that the displacement of certain pairs of the steering cables results in bending of the steering section in one plane or another. As disclosed in that patent application, the washers are flat washers, and the spacer structure includes pairs of hemispherical beads that are disposed in nose-to-nose fashion over the respective cables between successive washers. The beads have their spherical surfaces facing one another, and their flat surfaces facing outward against their associated washers.
The upshot of this construction is that when the steering section is bent, the spherical surfaces of the hemispherical beads should roll over one another to achieve smooth bending without a significant amount of slack or play in the cable.
Unfortunately, with this design, the hemispherical spacer beads tended to degrade in service over time. The basic reason for this is that the steering cable had to have a significantly smaller diameter than the diameter of the through-bore of the beads to achieve proper clearance for the cable. Because of the difference in diameters of the cable and the spacer bead through-bores, there is a tendency for the hemispherical spacer beads to shift off axis by the amount of the clearance. When the spacer beads rock over one another, the intersecting edges of the through-holes cut into one another in the radially displaced beads. This eventually works into a saddle and binds on the cable.
In order to avoid this problem, it was necessary to provide a relatively wide bearing surface at the facing noses of the beads to compensate for this lateral sliding of the spacers relative to one another. However, if the nose surface is wide enough to compensate for the entire clearance between the steering cable and the spacer bead through-bores, then as one spacer bead tilts with respect to another, the effective track length of the cable is increased. Since the actual cable length does not increase, the effect of this is to tighten the cable when the steering section is deflected, and thereby increase steering forces.
Of course, a smaller bearing nose surface could be employed if there were a smaller diametrical clearance between the cable and spacer bead, but in that case the cable would bind against the spacer beads when the spacers tilted with respect to each other.
A further approach to this problem is to employ pairs of tapered spacer beads that have a relatively wide base surface facing the associated washer and a relatively flat nose surface facing against the like nose surface of the other spacer bead. The beads each have an axial bore that is flared out towards the base, so that the bead has a small clearance over the steering cable at the nose surface, but a wide clearance at the base to prevent the spacer beads from binding on the cable.
This configuration, while an improvement over previous designs, does still have some tendency to bind on the cable. One reason for this is that the dual spacer beads will make two points of contact with the cable, that is, at the narrow nose portion of each bead. Also, the rocking motion of the conic beads on one another does produce a noticeable tightening of the control cable, although certainly a smaller amount of tightness than would be encountered with previous designs.
Still further, it is somewhat difficult to string the beads onto the cable when the endoscope steering section is being assembled, because for one of each pair of beads the cable must be threaded into the narrow nose portion of the through-bore.